Electroacoustical apparatus



Feb. 14, 1939. 1.. J. ANDERSON ELECTROACOUSTICAL APPARATUS Filed May 15,1936' 4 2 Sheets-Sheet'l INVEN'fOR J ANDERSON L59 BY Feb. 14, .1939. J.ANDERSON 2,147,137

I ELECTROACOUSTICAL APPARATUS Q Filed May 13, 1936' 2 Sheets-Sheet 2INVENT 5 OR LE .1 /0 5550 A TOR EY Patented Feb. 14, 1939 UNITED STATESELECTROACOUSTICAL APPARATUS Leslie J. Anderson, Westmont, N. J.,assignor to Radio Corporation of America, a corporation of DelawareApplication May 13,

15 Claims.

In such a microphone, a conductor of small mass, such as a conductiveribbon, for example, is disposed in a strong magnetic field to beactuated by sound wave pressure whereby an electromotive force is set uptherein which may be conveyed to suitable amplifying apparatus.Structures of this type heretofore employed havelbeen formed withrelatively wide ribbons, necessitating wide air gaps and large magnetsin order to provide desired sensitivity. I have found, however, that itis not necessary to have large, heavy, bulky microphones to obtain therequired sensitivity, and the primary object of my present invention isto provide an improvedmicrophone of the type set forth which is compactin construction.

Another object of my invention is to provide an improved microphone ofthe type aforesaid which is highly sensitive despite its compactness.

Still another object of my invention is to provide an improvedmicrophone of the type set forth wherein the conductive ribbon is somounted that it may be readily removed without disturbing the magneticcircuit.

A further object of my invention is to provide an improved microphone asaforesaid in which the conductive ribbon may be easily replaced byanother when necessary.

Still a further object of my invention is to provide an improvedmicrophone of the type set forth which will have good directionalcharacteristics.

It is also an-object of my invention to provide an improved microphoneas aforesaid which is simple in construction, which may be assembledwith great facility and therefore lends itself well to the requirementsof economical commercial manufacture, and which is highly efllcient inuse.

In accordance with my present invention, I reduce the width of the airgap considerably and use a narrower, although longer, conductive ribbonthan is conventionally used. Reducing the width of the air gap permitsreduction of the size of the magnetic structure mainly for two reasons.

In the first place, with a smaller air gap, the leakage flux isconsiderably less, and therefore the required cross section of themagnet steel is reduced; and in the second place, a shorter air gaprequires a correspondingly smaller amount of m2? gneto-motive force forthe same flux density. Since the magneto-motive force is a function ofthe length of the magnet (increasing withincreasing length of magnet),it is obvious that a shorter magnet will sufllce for a smaller air gap.

1936, Serial No. 79,434

The net result is that there is a. considerable reduction in the volumeof magnetic material required.

As stated above, the conductive ribbon is made longer than theconventional ribbon in order to obtain an equivalent output from myimproved microphone. The ribbon is preferably mounted on anon-conductive frame secured to the pole pieces and, in order to avoidany flimsiness of the relatively long, thin ribbon, I support the ribbonat substantially its center on the frame as well as at its .ends andthereby provide two ribbon sections. Since the two ribbon sections arein series, their voltages add and the resultant voltage is that of asingle ribbon having their combined length. The magnets are placed oneabove and one below the center line of the ribbon assembly and in backof it in a manner to provide best directional characteristics, theribbon and its frame being easily removable from the pole pieces withoutdisturbing the magnetic circuit when it is found necessary to replacethe ribbon.

The novel features that I consider characteristic of my invention areset forth with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation,together with additional objects and advantages thereof, will best beunderstood from the following description of a specific embodimentthereof when taken in connection with the accompanying drawings inwhich:

Figure 1 is a perspective view, on an enlarged scale for the sakeofclearness, of a microphone assembly in accordance with my presentinvention,

Figure 2 is a side elevation of the conductive ribbon supporting frameshowing the relation of the pole pieces thereto,

Figure 3 is a front elevation of the conductive ribbon supporting framewith the ribbon mounted thereon,

Figure 4 is a perspective view of the pole pieces showing themapproximately in their relation when assembled,

Figure 5 is a sectional view taken on the line VV of Figure 3, and

Figure 6 is a detail view showing the manner in which the microphoneassembly is swivelly mounted on the mounting yoke.

Referringmore specifically to the drawings. wherein similar referencecharacters designate corresponding parts throughout, there is shown amounting yoke I each arm of which is provided with a pair of apertures Iand 5 at its upper end (Fig. 6). The apertures 3 swivelly receive thereduced shanks I of a pair of horizontally aligned supporting blocks 9of magnetic material, such as soft iron, the outer ends of the blocks 9being threaded to receive the knurled thumb nuts II for locking theblocks 9 in desired position of adjustment, while the apertures receivethe offset lug I3 of a holding washer I5 for holding the microphoneassembly against easy removal from 10 the yoke I.

Mounted on the blocks 9 and clamped thereto bya pair of clamping framesl1 are two horseshoe magnets I9 and 2| each of which has a pole restingagainst each of the blocks 9. The magl5 nets l9 and 2| are disposed in acommon plane and in opposed physical relation, that is, the magnet I9rests on top of the blocks 9 and the magnet 2| rests against the bottomthereof, so that their respective north and south poles face no eachother. The blocks 9 extend laterally forward of the magnets I9 and 2|,and to each end of the lateral extensions thereof, there is secured .apole piece 23 the adjacent edges 24' of which are slightly spaced fromeach other (approxi- 25 mately 3%") to provide an air gap 25therebetween, as clearly shown in Fig.- 4. Thus, the pole pieces 23 arealigned in a common plane spaced from and parallel to that of themagnets l9 and 2|. It will be seen from the foregoing 3o descriptionthat the magnets I9 and 2|, the magbeing provided with a longitudinallyextending slot 29 bridged centrally by a bridge piece 3| which extendsthrough the slot 29 to provide a lug 33 (Fig. 5). The slot 29 is ofsubstantially the samejwidth as and is aligned with the air gap 25, andthe frame 21 is beveled off at 35 as it approaches the slot 29. Theframe 21 is also formed with a pair of flanges 31 along its upper andlower edges between which the pole pieces 23 fit, as clearly shown inFig. 2. The flanges 31 .have a thickness substantially equal to thethickness'of the lug 33, and a conductive element 39,

- sucn as a thin, preferably slightly crimped, ribbon of aluminum foilis secured at both its ends to the flanges 31 by means of conductivestrips 4| and the screws 43, and at its center to the lug 55 33 by meansof a strip and the screws 31.

- The ribbon 39, which lies in the air gap 25, ex-

tends the full length of the frame 21 and occupies therewith a'planesubstantially parallel to the magnets "I9, the latter of which are sodisposed 6o that'they are arranged symmetrically with respect to thecenter of the ribbon 39 or the bridge piece 3|, one above it and theother below it. I It will also be noted that the length of the frame f21 and the ribbon 39 is of the same order of 5 magnitude as the overalllength between the bases 5 of the magnets, being only slightly less thanthe overall length between the bases of the magnets l9.

The ribbon or conductive element 39, being 70 long and thin, isordinarily somewhat flimsy. It is'for thisgreason that it is clamped atits center to the lug 33. This forms two serially connected ribbonsections 39a and 39b each of which is quite flrm. Since the two ribbonsections are- 75 series connected, their voltages add, so that theribbon 39 may be connected to one end of the 5 primary coil of thetransformer by a lead 53, and the other end thereof is preferablyconnected to the other end of the primary coil by a pair of 15 Ioppositely extending leads 53 and 55 threaded through apertures in theframe 21, as clearly shown in Fig. 1. This affords a pair of opposedloops formed, in one case, by the ribbon 39, the lead 55, the primarycoil of the transformer, and 20 the lead 53, and in the other case bythe ribbon 39, the lead 51, the transformer primary, and the lead- 53.These loops eliminate distortion due to energy picked up inductively bythe microphone since the two loops balance each other out. The 2 usualscreened casing 59 and wind screen (not shown) may be placed about thestructure herein described to protect the partsfrom injury.

From the foregoing description, it will be apparent that I have provideda novel microphone 30 that is not only compact in construction, but iscapable of giving excellent performance. The simplicity of theconstruction enables easy assembly of the entire structure. Also, sincethe frame 21 and the ribbon 29 are assembled as a 35 unit and aresecured, as a unit, to the pole pieces 23, they can be removedphysically as a unit from the pole pieces for any correction oradjustment that may be necessary without disturbing the magnetic circuitat all. This further adds to the advantages of my improvedmicrophonefrom a commercial standpoint.

Although I have shown and described but a single embodiment of myinvention, I am fully aware that many modifications thereof arepossible. For example, in place of the horse-shoe magnets shown, asingle bar magnet may be placed between theblocks 9. If more than onebar magnet is used, the blocks maybe formed in the shape of a T placedon its side with the lgs of the Ts constituting'the lateral extensions,and the cross pieces thereof having one magnet at each end. .Othermodifications will, no doubt, readily suggest themselves to thoseskilled in the art. I desire, therefore, that my invention shall not belimited except insofaras is made necessary by the prior art and by thespirit of the appended claims.

' I claim as my invention:

1. In sound translating apparatus, a magnet 60 support of magneticmaterial, a magnet mounted thereon, said support extending laterally toone side of said magnet and a pole piece mounted on the end of thelateral extension of said support.

2. In sound translating apparatus, a pair of spaced supporting membersof magnetic material, a magnet mounted thereon with a pole thereof oneach of said members, said members extending laterally to one side ofsaid magnet, and a pole piece mounted on the end of the lateralextension of each of said members, said magnet supporting members andpole pieces combining to form a magnetic path.

3. In sound translating apparatus, a pair of spaced supporting membersof magnetic material,

a magnet mounted thereonwith a pole thereof on each of said members,said members extending laterally to one side of said magnet, a polepiece mounted on the lateral extension of each of said members andcombining with said members and said magnet to form a mag netic path,said pole piecesbeing slightly spacedfrom each other to provide an airgap. a frame of non-conductive materialmounted on said pole pieces, anda conductive element carried by said frame .and so arranged thereon asto lie in said air gap.

4. The invention set forth in claim 3 character ized in that saidconductive element is secured to said frame at both of its ends and at apoint intermediate its ends.

5. The invention set forth in claim 3 characterized ingthat saidconductive element comprises a ribbon lying substantially in a planeparallel to said magnet,

6. The invention set forth in claim 3 characterized in that saidconductive element comprises ,a ribbon lying substantially in a planeparallel to said magnet, and characterized further in that said ribbonis secured to said frame at both its- 7 ends and at a. pointintermediate its ends.

7. The invention set forth in claim 3 characterized in that .saidconductive element comprises a ribbon lying substantially in a planeparallel to said magnet, and characterized further in that said ribon issecured to said frame at both its ends and at a point substantiallymidway between its ends. I

, 8. In sound translating apparatus, a pair of spaced supporting membersof magnetic material,

upper and lower magnets mounted thereon with a pole -of each of saidmagnets on each of said members, said members extending laterally to oneside of said magnets, a pole piece mounted on the lateral extension ofeach of said members and combining with said supporting members and saidmagnets to form a magnetic path, said pole pieces being slightly spacedfrom each other to provide 10. The invention set forth in claim 8characterized in that said magnets are aligned with each other inacommon plane, and characterized further in that said conductive ribbonis disposed in a plane parallel to said magnets.

11. The invention set forth in claim 8 characterized in that saidmagnets are aligned with each other in a common plane, and characterizedfurther by the addition of means for securing said magnets to saidsupporting members in aligned relation.

12. The invention set forth in claim 8 characterized in that saidsupporting members comprise a pair of horizontally disposed blocks,characterized further in that said magnets are of the horseshoe typewith one of said magnets mounted on top of said blocks and the other onthe bottom of said blocks, and characterized still further by theaddition of means for securing said magnets to said blocks.

13. The invention set forth in claim 8 characterized in that saidsupporting members comprise a pair of horizontally disposed blocks,characterized further in that said magnets are of the horseshoe typewith one of said magnets mounted ontop of said blocks and the other onthe bottom of said blocks and with the poles of the respective magnetsfacing each other, and characterized still further in that said frameand said conductive element are on a plane parallel to said magnets,

said frame and conductive element having a length of the same order ofmagnitude as the overall length between the bases of said magnets.

14. In combination a yoke, a pair of supporting blocks of magneticmaterial swivelly mounted on said yoke one on'each arm of the yoke, apair of horse-shoe magnets mounted on said blocks in a common plane, apole of each of said magnets resting against each of said blocks, one ofsaid magnets being mounted on top of said blocks and the other on thebottom thereof in physically opposed relation, said blocks extendinglaterally from said magnets, a pole piece on the lateral extension-ofeach of said blocks and combining with said blocks and said magnets toform a magnetic path, said pole pieces lying in a common plane spacedfrom said magnets and having their adjacent'edges slightly spaced fromeach other to provide an air gap therebetween, a frame of nonconductivematerial mounted on said pole pieces, a conductive ribbon movablycarried by said frame and so arranged thereon as to lie in said air gapfor slight movement therein, a bracket member associated with said yoke,a casing carried by 'said bracket member, a transformer house in saidcasing, and leads connecting each end of said ribbon with the primarycoil of said transformer.

15. In a microphone, the combination of a frame member, and a thin,relatively long, narrow conductive ribbon carried thereby, said ribbonbeing secured to said frame member at both its ends and at a pointintermediate its ends whereby to provide a pair of serially connectedribbon sections.

LESLIE J. ANDERSON.

